The present invention relates to a video display having a picture-in-picture (PIP) function for simultaneously displaying two pictures, a main picture and a sub-picture, and more particularly to a video state stabilization method and apparatus for use in a video display having a picture-in-picture function in which the brightness of a sub-picture is not influenced by that of a main picture.
FIG. 1 shows a conventional apparatus for adjusting the video brightness when two pictures, that is, two channels, are simultaneously viewed on a single display, or when video signals supplied from a television and a video cassette recorder are simultaneously viewed on a single display.
FIG. 1 is a block diagram of a video display having a conventional PIP function. A video signal processor 11 receives a main video signal V.sub.M corresponding to a main picture and a sub-video signal V.sub.s corresponding to a sub-picture, respectively. Video signal processor 11 compresses sub-video signal V.sub.s into a signal appropriate to a sub-picture, and then replaces a portion of main video signal V.sub.M with sub-video signal V.sub.s, to simultaneously display the main picture and the sub-picture. Then, video signal processor 11 separates the combined video signal into a luminance signal Y, red color difference signal R-Y and blue color difference signal B-Y, and supplies the separated signals to video signal adjuster 15. Video signal adjuster 15 converts luminance signal Y and color difference signals R-Y and B-Y into primary color signals, R, G and B. Video signal adjuster 15 also adjusts the contrast, brightness, color density and color tone with respect to the input video signals in response to a control signal supplied from a controller 12 and an automatic beam limiting (ABL) control signal supplied from a beam current detector 14. An amplifier 16 amplifies R, G and B primary color signals supplied from the video signal adjuster 15 into a predetermined amplitude and then supplies the amplified signal to a cathode ray tube (CRT). On the other hand, a high voltage generator 13 uses a high frequency pulse generated in a deflection circuit (not shown) to generate a high voltage of 15-32 kV used as a power source of an anode in the picture tube. Here, beam current detector 14 detects a beam current flowing from the anode to a cathode in the CRT due to the high voltage generated in high voltage generator 13, and supplies an ABL control signal corresponding to the detected beam current to a video signal adjuster 15. The ABL control signal prevents an excessive load in high voltage generator 13 due to an excessive beam current flowing in the CRT. Further, the ABL control signal is used for adjusting characteristics such as the brightness and the contrast of a video signal supplied to the CRT. Beam current detector 14 outputs a low-level ABL control signal when a large beam current flows, while it outputs a high-level ABL control signal when a small beam current flows. Video signal adjuster 15 adjusts the contrast of the picture according to the level of the ABL control signal supplied from beam current detector 14. That is, video signal adjuster 15 heightens the contrast of the picture when a high-level ABL control signal is applied thereto, while it lowers the contrast of the picture when the low-level ABL control signal is applied thereto, thereby preventing damage to the CRT due to excessive beam current or preventing a saturation phenomenon of the picture due to excessive beam current.
However, since the contrast and/or the brightness of the sub-picture is forcedly controlled according to the contrast and/or the brightness of the main picture which occupies most of the picture, the brightness of the sub-picture is influenced by the brightness of the main picture when the difference in brightness between the two pictures is large. Thus, when the main picture is bright, the sub-picture is dark and the beam current is excessive, the video signal adjuster 15 lowers the contrast thereof to make the sub-picture grow darker. On the contrary, when the main picture is dark, the sub-picture is bright and the beam current is insufficient, the video signal adjuster 15 heightens the contrast thereof to cause the creation of a white color saturation phenomenon in the sub-picture.
In U.S. Pat. No. 5,204,748, an FS signal is used for discriminating a main picture and a subordinate picture. When the main picture is displayed, an output signal of a beam current limiter is used for controlling a contrast of a video signal, while when the subordinate picture is displayed, the contrast of the video signal is controlled according to a contrast value output from a receiver controller which is set by a user.